1. Field of the Invention
The invention relates to a fuel cell system provided with a fuel cell having a plurality of cells, and particularly to current limiting processing and current restoration processing while a system-request current is being controlled.
2. Background of the Invention
Among fuel cells for generating electrical power utilizing an electrochemical reaction between hydrogen and oxygen, polymer electrolyte fuel cells are commonly known. The polymer electrolyte fuel cell includes a stack which is constituted from a plurality of stacked cells. The cells constituting the stack each include an anode (fuel electrode) and a cathode (air electrode), and a solid polymer electrolyte membrane having a sulfonic acid group as an ion exchange group is interposed between each anode and cathode.
A fuel gas containing a hydrogen gas (hydrogen-enriched reformed hydrogen obtained by reforming hydrogen gas or hydrocarbon) is supplied to the anode, while an oxygen-containing gas (oxidant gas), e.g., air, as an oxidant is supplied to the cathode. Upon the supply of the fuel gas to the anode, hydrogen contained in the fuel gas reacts with a catalyst in a catalyst layer which constitutes the anode, thereby generating hydrogen ions. The generated hydrogen ions pass through the solid polymer electrolyte membrane and electrically react with oxygen in the cathode. Electrical power is thus generated through the electrochemical reaction.
In a situation where a fuel cell having a plurality of cells is used, when even only one of the plurality of cells becomes unable to generate electrical power, the entire fuel cell becomes unable to generate electrical power. Under such circumstances, for example, JP 2003-187842 A has proposed an invention in which the voltage of each cell is measured and the amount of electrical power that can be output by a fuel cell stack is calculated based on the lowest cell voltage having the lowest value in the measured cell voltages, while the fuel cell stack generates an electrical energy equal to or smaller than the amount of electrical power which can be output, so that, even when the performance of an arbitrary cell is degraded, the fuel cell stack can operate in a suitable operation state in accordance with the degradation of the performance of the cell.
In addition, JP 2005-197008 A has proposed an invention in which a target current-limit value for limiting a power generation current to be taken out from a fuel cell in accordance with the lowest cell voltage VLOW is set, and, when the difference between an actual current-limit value and the power generation current is below a predetermined value, the current-limit value is replaced with the target current-limiting value.
However, in conventional fuel cell systems, when electrical power to be generated by the fuel cell is controlled or a supply current to be supplied to a load is limited under certain conditions, a system-request power, being the reference for the limiting computation, in the fuel cell increases, and the fuel cell system might eventually become unable to perform limiting processing for the power to be generated or the supply current. A possible example would be a situation where, in an automobile equipped with such a fuel cell system, a power to be generated or a supply current is limited when a driver is trying to accelerate the automobile. When the power to be generated or a supply current is limited, the driver feels as if a motor output is insufficient and performs an operation such as further stamping on an accelerator. As a result of the increase in system-request power due to such an operation, the power to be generated or supply current also increases, and consequently necessary limiting processing cannot be performed.
Also, in conventional fuel cell systems, even though there is a difference between the response required in current limiting processing for reducing a supply current and the response required in current restoration processing for increasing the limited supply current, a common response has been set without taking the difference into account. In other words, if one of cell voltages falls below a lowest-cell-voltage acceptable value, a current value of an actual supply current is limited. Then the limited state is maintained until the relevant cell voltage increases and reaches such a acceptable value, and after the cell voltage reaches such a acceptable value, the supply current value is restored to the system-request current. At this time, the current limiting processing is required to rapidly limit a current; however since the current restoration processing has been controlled using the same response as in the current limiting processing, overshooting might occur in the supply current during the current restoration processing. On the other hand, if the response that will not cause overshooting in the current restoration processing is set, it is expected that the response will be poor at current limiting processing.    Patent document 1: JP2003-187842 A    Patent document 2: JP2005-197008 A